1) Field of the Invention
The present invention relates to an optical scanner and a tandem-type image formation apparatus.
2) Description of the Related Art
In a tandem-type image formation apparatus, four drum-shaped photosensitive members are arranged along a transfer path of a paper, those photosensitive members are optically scanned simultaneously to form an electrostatic latent image on the respective photosensitive members, the electrostatic latent images are separately visualized by four kinds of toners, that is, yellow, magenta, cyan, and black, and the color toner images are transferred onto the same paper and superposed, to thereby obtain a color image.
Such tandem-type image formation apparatus has been put into practical use as a digital copying machine, an optical printer, and the like, wherein a color image and a monochrome image can be formed at the same speed, thereby enabling high-speed formation of a color image. On the other hand, however, if an optical scanner is provided for each photosensitive member, the image formation apparatus disadvantageously becomes big.
Moreover, if a separate optical scanner is used for each photosensitive member, the size and the like of the respective color toner images superposed on the transfer paper may become slightly different, due to a slight difference in the optical characteristics of the optical scanners, and hence “out-of-color registration” is likely to occur in the obtained color image. Furthermore, if a separate optical scanner is used for each photosensitive member, and if an optical element (plastic lens or the like) made of resin is included in the optical system, temperature becomes different for each optical element made of resin due to a temperature rise in the apparatus, at the time of continuously operating the image formation apparatus. Therefore, the optical characteristics of each resin optical element become nonuniform, and as a result, the hue of the output color image may change with the lapse of time.
One approach to overcome these problems is to share an optical deflection unit, which deflects the optical beam, by the four photosensitive members. This technique has been disclosed in, for example, Japanese Patent Application Laid-open No. 2001-4948, Japanese Patent Application Laid-open No. 2001-10107, and Japanese Patent Application Laid-open No. 2001-33720.
The image formation apparatus that includes one optical deflection unit is called as a four-drum one-sided deflection type apparatus, since the whole optical beams for optically scanning the four photosensitive members are deflected all by one side of the optical deflection unit.
The four-drum one-sided deflection type apparatus is compact since a part of the optical deflection unit and the scanning image formation optical systems in the optical scanner is shared by a plurality of photosensitive members as compared with an instance when a separate optical scanner is used for each photosensitive member.
Further, because a part of the scanning image formation optical system is shared by a plurality of optical beams, a difference in the optical characteristics in each scanning image formation optical system can be reduced. When the “shared optical element” is made of resin, even if the optical characteristics of the “shared optical element” changes due to a temperature change or the like, this change is common to “a plurality of sharing optical beams”. As a result, the “out-of-color registration” and “hue change with the lapse of time” at the time of continuous operation can be effectively reduced.
FIG. 1 illustrates the main parts of the image formation apparatus of the four-drum one-sided deflection type apparatus. Four optical beams irradiated from a light source (not shown) are made to fall onto a polygon mirror PM, which is the optical deflection unit. These four optical beams are substantially parallel luminous flux, and enter from a direction substantially orthogonal to the rotation axis of the deflection reflecting surface of the polygon mirror PM, and are also deviated in the vertical scanning direction (in the direction of the rotation axis) at an equal interval.
The four optical beams reflected and deflected by the deflection reflecting surface of the polygon mirror PM pass through a lens L1, and are optically guided to photoconductive photosensitive members Y, M, C, and Bk formed in a drum shape, with the optical path thereof optically separated by mirrors M1 to M8.
The photosensitive member Y is for forming an electrostatic latent image to be visualized by the yellow toner, and the optical beams optically guided by the mirrors M1 and M5 are imaged as a light spot, by the action of lenses L1 and L2Y. The photosensitive member M is for forming an electrostatic latent image to be visualized by the magenta toner, and the optical beams optically guided by the mirrors M2 and M6 are imaged as a light spot, by the action of lenses L1 and L2M.
Likewise, the photosensitive member C is for forming an electrostatic latent image to be visualized by the cyan toner, and the optical beams optically guided by the mirrors M3 and M7 are imaged as a light spot, by the action of lenses L1 and L2C. The photosensitive member Bk is for forming an electrostatic latent image to be visualized by the black toner, and the optical beams optically guided by the mirrors M4 and M8 are imaged as a light spot, by the action of lenses L1 and L2B.
In this optical scanner, if the size of the desired optical scanning area is about 300 millimeters, and the spot diameter of the light spot formed on the respective photosensitive members to not larger than 50 micrometers, the diameter of luminous flux of the respective optical beams entering into the polygon mirror PM becomes about 5 millimeters. In this case, because it is necessary to separate the respective optical beams after having transmitted through the lens L1, the optical beams should not overlap on each other in the direction of rotation axis of the polygon mirror PM. Therefore, the deflection reflecting surface of the polygon mirror PM should be about 17 millimeters, as shown in the figure, in the direction of rotation axis.
The polygon mirror having the deflection reflecting surface of 17 millimeters in the direction of rotation axis has a large so-called “windage loss” at the time of high-speed rotation, thereby causing noise and an increase in the power consumption and fluctuation. Hence, there are many problems in speed-up of write itself.
As an optical arrangement that makes it possible to decrease the deflection reflecting surface of the polygon mirror in the direction of rotation axis, one shown in FIG. 2 can be considered. In order to avoid confusions, parts, which are considered not to be confusing, are designated by like reference signs as in FIG. 1.
In the example in FIG. 2, the four optical beams falling onto a polygon mirror PM1 are inclined with respect to the surface orthogonal to the rotation axis of the deflection reflecting surface of the polygon mirror PM1, and the angle of inclination is made to be different for each optical beam. In this case, because the four optical beams can be made to enter into the “same position in the direction of rotation axis” of the deflection reflecting surface of the polygon mirror PM1, the size of the deflection reflecting surface of the polygon mirror can be made smaller. In this polygon mirror PM1, the windage loss resulting from high-speed rotation is small, and hence high-speed rotation is possible with low power consumption, and high-speed write is also possible.
In the optical arrangement illustrated in FIG. 2, however, if an angular difference between the optical beams at the position of the deflection reflecting surface is small, it is necessary to arrange the mirrors M1 to M4 at a longer distance from the lens L1 to avoid an optical beam of one mirror falling on other mirror. As a result, the optical scanner becomes bulky.
On the contrary, if the angular difference is large, the optical scanner can be made compact, however, “skew of beams” occurs in the optical beams due to the large angle of inclination of the deflected optical beams, thereby deteriorating wave front aberration. If the wave front aberration deteriorates, it becomes difficult to achieve a small diameter of the light spot. For example, in the optical arrangement in FIG. 2, if the angle of inclination of the optical beams is about five degrees, even with the optical beams having a diameter of luminous flux of 5 millimeters in the same configuration as in FIG. 1, the spot diameter of the formed light spot is limited to about 60 micrometers, designating 1/e2 of the intensity of light intensity distribution as a threshold.
With the light spot having such a size, sufficient resolution and tone cannot be obtained with respect to an image having a dot density of for example 1200 dots per inch (24.1 micrometer pitch).
Separation of the deflection reflecting surface of the polygon mirror into two stages is described in Japanese Patent Application Laid-open No. 2002-278207 relating to one-drum type color image formation apparatus. Further, electrical correction of constant velocity characteristics as being performed in the exemplary embodiments in the present invention is described in Japanese Patent Application Laid-open No. 2000-36625.